Light Emitting Diodes (LEDs) are increasingly being adopted as general illumination lighting sources due to their high energy efficiency and long service life relative to traditional sources of light such as incandescent, fluorescent and halogen. Each generation of LEDs are providing improvements in energy efficiency and cost per lumen, thus allowing for lighting manufacturers to produce LED light fixtures at increasingly cost competitive prices.
While the component LEDs are increasing in energy efficiency, there are considerable other components within an LED light fixture that cause drains on the overall lumens per Watt achievable within the overall system. For instance, inefficiencies within the optics used and the AC/DC power supply both reduce the overall energy efficiency for the light fixture. Further, operating the LEDs at non-optimal current (amperage) levels can reduce the energy efficiency of the LEDs and therefore the overall light fixture. In fact, if the current flowing through the LEDs exceeds a maximum allowable level, the LEDs may be damaged or permanently burnt out, and thus require replacement.
One issue that arises when managing the current flow through the LEDs is that LEDs are not constant in terms of their forward voltages (voltage drops), or other characteristics such as color/color temperature and brightness. Each LED that is manufactured is generally categorized based on a number of criteria including their forward voltage, this categorization being based on bin codes and therefore called “binning” In manufacturing light fixtures with LEDs, the developer will generally want a consistent light output from each of its “identical” light fixtures. To do this, the manufacturer will select the binning of its LEDs to be consistent. A problem that arises is that the more narrow the binning criteria provided by the developer, the higher prices that are typically charged for the LEDs. On the other hand, it becomes difficult to design a circuit for the LEDs if the uncertainty related to the forward voltages of the LEDs is significant.
In some architectures, the LED light fixtures are powered with a constant current power supply that can be locked to a particular current through a feedback control system using an inductor. In this architecture, the control system can manage differences in forward voltages within the LEDs within a predefined limit.
In other architectures, a constant voltage power supply is used and the current to flow through the LEDs is controlled by the voltage output from the voltage power supply and the impedances within the circuit. In this case, differences in forward voltages across LEDs within different light fixtures can make a significant difference in the current flowing through the LEDs. Further, the voltage output from the constant voltage power supply may be adjustable and adjustments in the voltage supply can further change the current flowing through the LEDs.
Against this background, there is a need for solutions that will mitigate at least one of the above problems and, in particular, to enable monitoring and/or control of the current flowing through the LEDs while not materially decreasing the efficiency of the system.